Conformal coatings comprising carbon nanotubes

ABSTRACT

The invention is directed to conformal coatings that provide excellent shielding against electromagnetic interference (EMI). A conformal coating comprises an insulating layer and a conducting layer containing electrically conductive material. The insulating layer comprises materials for protecting a coated object. The conducting layer comprises materials that provide EMI shielding such as carbon black, carbon buckeyballs, carbon nanotubes, chemically-modified carbon nanotubes and combinations thereof. The insulating layer and the conductive layer may be the same or different, and may be applied to an object simultaneously or sequentially. Accordingly, the invention is also directed to objects that are partially or completely coated with a conformal coating that provides EMI shielding.

REFERENCE TO RELATED APPLICATIONS

The application claims priority to U.S. Provisional Application No.60/307,885, entitled “EMI Shielding with Carbon Nanotubes,” filed Jul.27, 2001.

GOVERNMENT INTEREST

This invention was made, in part, with support for the United Stategovernment under a grant from the U.S. Army, SBIR No. DAAH01-01-C-R098,and the U.S. Government has certain rights to this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The instant invention relates to a conformal coating that providesexcellent shielding against electromagnetic interference (EMI). Moreparticularly, the instant invention relates to conformal coatingcomprising an insulating layer and a conducting layer. It isparticularly preferred that the conducting layer of the instantinvention comprises carbon nanotubes.

2. Description of Background

The demand for electronic assemblies in the automotive, aerospace andvarious other industries has resulted in the annual production ofmillions of electronic assemblies by manufacturers in the electronicsindustry. Often, demand has increased to the point that additionalprocessing equipment and floor space is required to meet the growingdemand. To enhance their production efficiencies, electronicsmanufacturers continuously seek to implement new technologies which canincrease output without a corresponding increase in capital, floor spaceand labor.

As is also well known in the art, electronic assemblies are oftenrequired to be capable of withstanding hostile operating environments,such as those commonly found in the automotive and aerospace industries.One practice widely accepted in the electronics industry is the use of aconformal coating which forms a protective barrier layer on the circuitboard. Conformal coatings are formulated to protect the electronicassembly from moisture and dirt, as well as make the circuit devicesmounted to the circuit board more resistant to vibration.

Specifically, a conformal coating is a thin layer of insulating materialwith a consistent thickness that closely conforms to the shape andcontours of the entire substrate, such as a circuit board. Thispolymeric covering shields metallic junctions and sensitive componentsfrom the deleterious effect of the environment. When properly applied,the coating is a barrier against adverse hazards like dust and dirt,moisture, harsh solvents, high atmospheric humidity, airborne chemicalvapors and environmental contaminants. Contamination can compromise acircuit board's operational life. When bare circuits are exposed tohumid air, thick films of water molecules can form on their surfaces.Moisture on uncoated circuits can induce metallic growth and corrosion.The thicker the water film, the lower the surface insulation resistance,and the greater the effects on electrical signal transmission. This canresult in cross talk, electrical leakage from high-impedance circuitsand intermittent transmission, all leading to diminished and oftenterminated circuit performance. Conformal coatings permit closer circuittraces and decreased line spacing popular with high-component densitiesand minimizing shorts from bridging. Conformal coatings supportcomponents so the solder joints do not carry the entire mass of thecomponent. Many coatings can even improve resistance to abrasion,thermal shock and vibration. These coatings are essential in automotiveassemblies, industrial controls domestic appliances, certain consumerproducts, military and aerospace systems, and medical devices.

Conformal coatings find application in device components of cell phonesand computers, particularly integrated circuits, printed wire boards,and printed circuit boards.

Generally, conformal coatings have been composed of polymeric materialsof the silicone, acrylic, urethane and epoxy families. These familiescan be divided into groups based on their particular systems and theircuring characteristics. For example, there are two-part material systemswhich cure upon mixing of the two components, one-part solvent-bornesystems such as acrylic and hydrocarbon resins, one-part moisture curesystems, such as urethanes, epoxies and silicones, one-part frozenpremixed systems, one-part heat-cured systems, ultraviolet (UV) curedsystems, and vacuum deposited materials such as parylene, availablethrough the Union Carbide Corporation.

Other than the vacuum depositing materials, the above coating systemsare typically applied by dipping, spraying or brushing techniques, andoccasionally are deposited as multiple layers. The product design, thecoating process and the process capacity will generally dictate whichtype of coating system can-be applied for a given application.

In recent years a need has developed, particularly in the aerospaceindustry and in military applications, for electrical connectorsproviding effective shielding against electromagnetic interference (EMI)and, in certain applications, having the ability to withstand severe EMIconditions.

It is widely understood that highly conducting materials provide EMIshielding. The development of intrinsically conductive organic polymersand plastics has been ongoing since the late 1970's. These efforts haveyielded conductive materials based on polymers such as polyanaline,polythiophene, polypyrrole, and polyacetylene (See “ElectricalConductivity in Conjugated Polymers.” Conductive Polymers and Plasticsin Industrial Applications”, Arthur E. Epstein; “Conductive Polymers.”Ease of Processing Spearheads Commercial Success; Report from TechnicalInsights; Frost & Sullivan; “From Conductive Polymers to OrganicMetals.” Chemical Innovation, Bernhard Wessling).

A significant discovery was that of carbon nanotubes, which areessentially single graphite layers wrapped into tubes, either singlewalled nanotubes (SWNTs) or double walled (DWNTs) or multi walled(MWNTs) wrapped in several concentric layers (B. I. Yakobson and R. E.Smalley, “Fullerene Nanotubes: C_(1,000,000) and Beyond”, AmericanScientist v.85, July-August 1997). Although only first widely reportedin 1991 (Phillip Ball, “Through the Nanotube”, New Scientist, 6 Jul.1996, p. 28-31), carbon nanotubes are now readily synthesized in gramquantities in the laboratories all over the world, and are also beingoffered commercially. The tubes have good intrinsic electricalconductivity and have been used in conductive materials.

Heretofore, conformal coating comprising carbon nanotubes that providesEMI shielding has not been disclosed.

SUMMARY OF THE INVENTION

Accordingly, the invention provides, in a preferred embodiment, aconformal coating that provides EMI shielding, wherein said coatingcomprises: an insulating layer; and a conductive layer which may bedisposed on or combined with the insulating layer, wherein theconductive layer comprises a electrically conductive material.Accordingly, the insulating layer and the conductive layer may be thesame or different. The electrically conductive material may comprisecarbon black, an electrically conductive metal such as, for example,nickel, silver or copper, carbon nanotubes, chemically- orstructurally-modified carbon nanotubes, or a combination thereof.

In another preferred embodiment, the invention provides a coatedsubstrate with EMI shielding comprising: a substrate; and a conformalcoating disposed on said substrate that provides EMI shielding, whereinsaid coating comprises: an insulating layer; and a nanotube-containinglayer disposed on said insulating layer, wherein saidnanotube-containing layer comprises a plurality of carbon nanotubes.Preferably, the EM shielding is from 5-70 dB or greater.

In another preferred embodiment, the invention provides a method forimparting EMI shielding to a substrate, comprising coating saidsubstrate with a conformal coating wherein said conformal coatingcomprises: an insulating layer; and a nanotube-containing layer disposedon said insulating layer, wherein said nanotube-containing layercomprises a plurality of carbon nanotubes.

In another preferred embodiment, the invention provides a conformalcoating that provides EMI shielding, wherein said coating comprises aplurality of carbon nanotubes and a polymer selected from the groupcomprising acrylics, epoxies, silicone, polyurethane, and parylene.

In another preferred embodiment, the invention provides a dispersioncomprising a plurality of carbon nanotubes and a conformal coatingmaterial selected from the group comprising polyurethanes, parylene,acrylics, epoxies and silicone.

Other embodiments and advantages of the invention are set forth, inpart, in the following description, will be obvious from thisdescription, or may be learned from the practice of the invention.

DESCRIPTION OF THE INVENTION

While the invention is described and disclosed here in connection withcertain preferred embodiments, the description is not intended to limitthe invention to the specific embodiments shown and described here, butrather the invention is intended to cover all alternative embodimentsand modifications that fall within the spirit and scope of the inventionas defined by the claims included herein as well as any equivalents ofthe disclosed and claimed invention.

Conformal coatings of the instant invention protect device componentsfrom moisture, fungus, dust, corrosion, abrasion, and otherenvironmental stresses. The instant coatings conform to virtually anyshape such as crevices, holes, points, sharp edges and points, and flat,exposed surfaces.

In general, the invention is directed to the discovery that conformalcoatings impart excellent EMI shielding. In general, the instantconformal coatings comprise an insulating layer and a layer comprisingconducting materials. Alternatively, the insulating and the conductinglayer may be the same.

The instant conformal coatings provide EMI shielding properties in the10-70 dB attenuation range. Preferably, the instant coatings provide EMIshielding in the 20-70 dB attenuation range. Even more preferably, theinstant coatings provide EMI shielding in the 30-70 dB attenuationrange. Even still more preferably, the instant coatings provide EMIshielding in the 40-70 dB attenuation range.

Preferably, the insulating layer is a layer that comprises conventionalconformal coatings. Generally, the insulating layer may comprise aconformal coating material. Preferably, the insulating layer comprises amaterial selected from any known conformal coating, such aspolyurethanes, parylene, acrylics, epoxy and silicone. The advantages ofthese preferred conformal coatings are given below:

Type AR (acrylic)—Acrylics are easy to apply conformal coatings that arenot that resistant to abrasions and chemicals.

Type ER (epoxy)—Epoxies are fairly easy to apply and very hard toremove.

Type SR (silicone)—Silicone conformal coatings are for high temperatureenvironments.

Type UR (urethanes)—Polyurethane's are the most popular conformalcoatings, offering humidity, chemical and abrasion protection.

Type XY (parylene)—is a vacuum deposited conformal coating. They offerexcellent resistance to humidity, moisture, abrasion, high temperaturesand chemicals.

The conducting layer may comprise any electrically conductive material,including, but not limited to carbon black or metals such as silver,nickel, or copper.

In a preferred embodiment, the conducting layer comprises carbonnanotubes. It has surprisingly been discovered that carbon nanotubesimpart EMI shielding to conformal coatings. In general, carbon nanotubescan exhibit electrical conductivity as high as copper, thermalconductivity as high as diamond, strength 100 times greater than steelat one sixth the weight, and high strain to failure. However,heretofore, there has been no report of carbon nanotubes in a conformalcoating. Films comprising nanotubes are disclosed in U.S. patentapplication Ser. No. 10/105,623, entitled COATINGS COMPRISING CARBONNANOTUBES AND METHODS FOR FORMING THE SAME, filed Mar. 26, 2002, thedisclosure of which is incorporated herein by reference in its entirety.

Nanotubes are known and have a conventional meaning (R. Saito, G.Dresselhaus, M. S. Dresselhaus, “Physical Properties of CarbonNanotubes,” Imperial College Press, London U.K. 1998, or A. Zettl“Non-Carbon Nanotubes” Advanced Materials, 8, p. 443 (1996)).

In a preferred embodiment, nanotubes of this invention comprisesstraight and bent multi-walled nanotubes (MWNTs), straight and bentdouble-walled nanotubes (DWNTs) and straight and bent single-wallednanotubes (SWNTs), and various compositions of these nanotube forms andcommon by-products contained in nanotube preparations such as describedin U.S. Pat. No. 6,333,016 and WO 01/92381, which are incorporatedherein by reference in their entirety.

In another preferred embodiment, it has been discovered that nanotubeswith an outer diameter of less than 3.5 nm are preferred to impartconductivity and EMI shielding to conformal coatings.

The nanotubes of the instant invention have an outer diameter of lessthan 3.5 nm. In another preferred embodiment, nanotubes of the instantinvention have an outer diameter of less than 3.25 nm. In anotherpreferred embodiment, nanotubes of the instant invention have an outerdiameter of less than 3.0 nm. In another preferred embodiment, thenanotubes have an outer diameter of about 0.5 to about 2.5 nm. Inanother preferred embodiment, the nanotubes have an outer diameter ofabout 0.5 to about 2.0 nm. In another preferred embodiment, thenanotubes have an outer diameter of about 0.5 to about 1.5 nm. Inanother preferred embodiment, the nanotubes have an outer diameter ofabout 0.5 to about 1.0 nm.

The aspect ratio may be between 1.5 and 2,000,000, preferably between 10and 20,000, and more preferably between 15 and 2,000.

In a preferred embodiment, the nanotubes comprise single walledcarbon-nanotubes (SWNTs). SWNTs can be formed by a number of techniques,such as laser ablation of a carbon target, decomposing a hydrocarbon,and setting up an arc between two graphite electrodes. For example, U.S.Pat. No. 5,424,054 to Bethune et al. describes a process for producingsingle-walled carbon nanotubes by contacting carbon vapor with cobaltcatalyst. The carbon vapor is produced by electric arc heating of solidcarbon, which can be amorphous carbon, graphite, activated ordecolorizing carbon or mixtures thereof. Other techniques of carbonheating are discussed, for instance laser heating, electron beam heatingand RF induction heating. Smalley (Guo, T., Nikoleev, P., Thess, A.,Colbert, D. T., and Smally, R. E., Chem. Phys. Lett. 243: 1-12 (1995))describes a method of producing single-walled carbon nanotubes whereingraphite rods and a transition metal are simultaneously vaporized by ahigh-temperature laser. Smalley (Thess, A., Lee, R., Nikolaev, P., Dai,H., Petit, P., Robert, J., Xu, C., Lee, Y. H., Kim, S. G., Rinzler, A.G., Colbert, D. T., Scuseria, G. E., Tonarek, D., Fischer, J. E., andSmalley, R. E., Science, 273: 483-487 (1996)) also describes a processfor production of single-walled carbon nanotubes in which a graphite rodcontaining a small amount of transition metal is laser vaporized in anoven at about 1,200° C. Single-wall nanotubes were reported to beproduced in yields of more than 70%. U.S. Pat. No. 6,221,330, which isincorporated herein by reference in its entirety, discloses methods ofproducing single-walled carbon nanotubes which employs gaseous carbonfeed stocks and unsupported catalysts.

SWNTs are very flexible and naturally aggregate to form ropes of tubes.The formation of SWNT ropes in the coating or film allows theconductivity to be very high, while loading is very low, and results ina good transparency and low haze.

In another preferred embodiment, the nanotubes may be chemicallymodified, such as treatment with acid, base, or other reagent.

For example, acid treatment of nanotubes may be accomplished by blendingin the appropriate amount of nanotubes in a mixture of concentratedacid, such as nitric or concentrated sulfuric acid. This mixture may beheated at 70° C. without stirring for approximately 30 minutes. After 30minutes of heating, the reaction mixture is cooled and centrifuged. Thesupernatant liquid is then decanted and the functionalized nanotubes arewashed several times with water until a neutral pH is achieved.

After the tubes are functionalized they can be readily solvent exchangedand combined with conformal coatings or further derivatized byconventional synthetic techniques to incorporate other functionalgroups, moieties or residues.

Accordingly, in a preferred embodiment, the nanotube-containing layerhas a surface resistance in the range of less than about 10⁴ohms/square. Preferably, the nanotube-containing layer has a surfaceresistance in the range of about 10⁻²-10⁴ ohms/square. Even morepreferably, the film has a surface resistance in the range of less thanabout 10³ ohms/square. Even more preferably still, the film has asurface resistance in the range of less than about 10² ohms/square.Still even more preferably, the film has a surface resistance in therange of about 10⁻²-10⁰ ohms/square.

The instant nanotube-containing layer also has volume resistances in therange of about 10⁻² ohms-cm to about 10⁴ ohms-cm. The volume resistancesare as defined in ASTM D4496-87 and ASTM D257-99.

The nanotube-containing layer should demonstrate excellent transparencyand low haze. For example, the instant film has a total transmittance ofat least about 60%.

In a preferred embodiment, the nanotube-containing layer has a totallight transmittance of about 80% or more. In another preferredembodiment, the nanotube-containing layer has a total lighttransmittance of about 85% or more. In another preferred embodiment, thenanotube-containing layer has a total light transmittance of about 90%or more. In another preferred embodiment, the nanotube-containing layerhas a total light transmittance of about 95% or more. In anotherpreferred embodiment, the nanotube-containing layer has a haze valueless than 1%. In another preferred embodiment, the nanotube-containinglayer has a haze value less than 0.5%.

Total light transmittance refers to the percentage of energy in theelectromagnetic spectrum with wavelengths less than 1×10⁻² cm thatpasses through the films, thus necessarily including wavelengths ofvisible light.

The nanotube-containing layer range from moderately thick to very thin.For example, the second layer can have a thickness between about 0.5 nmto about 1,000 microns. In a preferred embodiment, thenanotube-containing layer can have a thickness between about 0.005 toabout 1,000 microns. In another preferred embodiment, thenanotube-containing layer has a thickness between about 0.05 to about500 microns. In another preferred embodiment, the nanotube-containinglayer has a thickness between about 0.05 to about 500 microns. Inanother preferred embodiment, the nanotube-containing layer has athickness between about 0.05 to about 400 microns. In another preferredembodiment, the nanotube-containing layer has a thickness between about1.0 to about 300 microns. In another preferred embodiment, thenanotube-containing layer has a thickness between about 1.0 to about 200microns. In another preferred embodiment, the nanotube-containing layerhas a thickness between about 1.0 to about 100 microns. In anotherpreferred embodiment, the nanotube-containing layer has a thicknessbetween about 1.0 to about 50 microns.

In another preferred embodiment, the nanotube-containing layer furthercomprises a polymeric material. The polymeric material may be selectedfrom a wide range of natural or synthetic polymeric resins. Theparticular polymer may be chosen in accordance with the strength,structure, or design needs of a desired application. In a preferredembodiment, the polymeric material comprises a material selected fromthe group comprising thermoplastics, thermosetting polymers, elastomersand combinations thereof. In another preferred embodiment, the polymericmaterial comprises a material selected from the group comprising ofpolyethylene, polypropylene, polyvinyl chloride, styrenic, polyurethane,polyimide, polycarbonate, polyethylene terephthalate, cellulose,gelatin, chitin, polypeptides, polysaccharides, polynucleotides andmixtures thereof. In another preferred embodiment, the polymericmaterial comprises a material selected from the group comprising ceramichybrid polymers, phosphine oxides and chalcogenides.

In another preferred embodiment, the polymer is a conformal coatingmaterial selected from the group comprising polyurethanes, parylene,acrylics, epoxies and silicone.

In another preferred embodiment, nanotube-containing layer isover-coated with a polymeric layer. Preferably, the polymeric layercomprises a conformal coating material selected from the groupcomprising polyurethanes, parylene, acrylics, epoxies and silicone.

The nanotube-containing layer may be easily formed and applied as adispersion of nanotubes alone in such solvents as acetone, water,ethers, alcohols, and combinations thereof. There are several methods toapply conformal coating to substrates. Some of the methods are typicallyperformed manually while others are automated. Selection of the coatingmethod and material is dependent on the environmental conditions thesubstrate will be exposed to during normal operation.

One of the preferred methods of coating is the dip process. The dipprocess can be done manually or automatically. In the manual modeoperators immerse the substrate in a tank of coating material. Thecoated parts are then hung to dry. The automatic dip systems consist ofa tank of coating material and a conveyer to move the substrate. Thesubstrates are placed on hangers that convey them to the tank, thenthrough the coating material, and then removed. Like the manual dipmethod the coated parts are then hung to dry. The advantages of thissystem are low capital investment, simplicity, and high throughput.

Brushing the material on the substrate is another preferred method ofapplication. This is a manual process where the operator dips a brushinto a container of coating material and brushes the material onto thesubstrate. The advantages of this system are no equipment investment, notooling or masking required, and the process is simple.

Manually spray painting is another common method used to apply conformalcoatings to substrates. In this method the operator can easily spray thesubstrate with a hand held sprayer gun similar to those used to spraypaint. The freshly spray coated boards are then allowed to cure prior toremoval. Like dip coating and brush application the advantages of thissystem are low capital investment, simplicity, and limited tooling.

Needle dispensing can either be done by hand or by an automated process.A simple tool valve can be used to turn material on and off. Thematerial is forced through a needle and is dispensed as a bead. Thebeads are strategically placed on the board allowing the material toflow and coat the appropriate area. The advantages of this system arelow equipment investment, no tooling or masking required, and theprocess is simple.

Conformal coatings can also be selectively applied using a dispensermounted to a robot. The robot is programmed to move and dispense coatingmaterial in designated locations on the substrate. The above process caneither be manually or conveyer loaded. The advantages of this system areconsistent application of material, high throughputs, no custom tooling,material saving, closed fluid system, and no masking. Manufacturers ofprinted circuit boards are continually faced with higher cost relatedmaterial and process decisions when addressing environmental impact ofelectronic assembly. In addressing the trade-off associated withregulatory compliance manufacturers strive for a contemporary approachto overall process savings in a synergistic fashion. To this end,manufacturers have turned to selective application of coating material.

The method for curing a conformal coating depends on the type of coatingused. The most commonly used methods are heat, UV, chemical reaction(moisture cure, free radical polymerization, etc.), or a combination ofany of the above-mentioned techniques.

The instant conformal coatings may be in a number of different formsincluding, but not limited to, a solid film, a partial film, a foam, agel, a semi-solid, a powder, or a fluid.

The instant nanotube-containing layers comprising nanotubes in a properamount mixed with a polymer can be easily synthesized. At most a fewroutine parametric variation tests may be required to optimize amountsfor a desired purpose. Appropriate processing control for achieving adesired array of nanotubes with respect to the plastic material can beachieved using conventional mixing and processing methodology, includingbut not limited to, conventional extrusion, multi-dye extrusion, presslamination, etc. methods or other techniques applicable to incorporationof nanotubes into a polymer.

The nanotubes may be dispersed substantially homogeneously throughoutthe polymeric material but can also be present in gradient fashion,increasing or decreasing in amount (e.g. concentration) from theexternal surface toward the middle of the material or from one surfaceto another, etc. Alternatively, the nanotubes can be dispersed as anexternal skin or internal layer thus forming interlaminate structures.

In a preferred embodiment, the instant nanotube films can themselves beover-coated with a polymeric material. In this way, the inventioncontemplates, in a preferred embodiment, novel laminates ormulti-layered structures comprising films of nanotubes over coated withanother coating of an inorganic or organic polymeric material. Theselaminates can be easily formed based on the foregoing procedures and arehighly effective for distributing or transporting electrical charge. Thelayers, for example, may be conductive, such as tin-indium mixed oxide(ITO), antimony-tin mixed oxide (ATO), fluorine-doped tin oxide (FTO),aluminum-doped zinc oxide (FZO) layer, or provide UV absorbance, such asa zinc oxide (ZnO) layer, or a doped oxide layer, or a hard coat such asa silicon coat. In this way, each layer may provide a separatecharacteristic.

In a preferred embodiment, the nanotubes are oriented by exposing thefilms to a shearing, stretching, or elongating step or the like (e.g.using conventional polymer processing methodology). Such shearing-typeprocessing refers to the use of force to induce flow or shear into thefilm, forcing a spacing, alignment, reorientation, disentangling etc. ofthe nanotubes from each other greater than that achieved for nanotubessimply formulated either by themselves or in admixture with polymericmaterials. Oriented nanotubes are discussed, for example in U.S. Pat.No. 6,265,466, which is incorporated herein by reference in itsentirety. Such disentanglement etc. can be achieved by extrusiontechniques, application of pressure more or less parallel to a surfaceof the composite, or application and differential force to differentsurfaces thereof, e.g., by shearing treatment by pulling of an extrudedplaque at a variable but controlled rate to control the amount of shearand elongation applied to the extruded plaque. It is believed that thisorientation results in superior properties of the film, e.g., enhancedelectromagnetic (EM) shielding.

Oriented refers to the axial direction of the nanotubes. The tubes caneither be randomly oriented, orthogonoly oriented (nanotube arrays), orpreferably, the nanotubes are oriented in the plane of the film.

In a preferred embodiment, the invention contemplates a plurality ofdifferentially-oriented nanotube-containing layer wherein each layer canbe oriented and adjusted, thus forming filters or polarizers.

In another preferred embodiment, the instant invention provides adispersion comprising a plurality of carbon nanotubes and a conformalcoating material. The conformal coating material may include one or moreof polyurethanes, parylene, acrylics, epoxies or silicone. The nanotubesmay be single-walled nanotubes (SWNTs), double-walled nanotubes (DWNTs),multi-walled nanotubes (MWNTs), and mixtures thereof. Preferably, thenanotubes are substantially single-walled nanotubes (SWNTs). The instantdispersion may comprise an a polymeric material comprisingthermoplastics, thermosetting polymers, elastomers, conducting polymersand combinations thereof. The dispersion may further comprise aplasticizer, softening agent, filler, reinforcing agent, processing aid,stabilizer, antioxidant, dispersing agent, binder, a cross-linkingagent, a coloring agent, a UV absorbent agent, or a charge adjustingagent. The instant dispersion may further comprise a conductive organicparticles, inorganic particles or combinations or mixtures thereof, suchas buckeyballs, carbon black, fullerenes, nickel, silver, copper andcombinations thereof. Preferably, the dispersion can form a coating,wherein the coating provides EMI shielding properties in the 10-70 dBattenuation range.

The present invention, thus generally described, will be understood morereadily by reference to the following examples, which are provided byway of illustration and are not intended to be limiting of the presentinvention.

Although only a few exemplary embodiments of the present invention havebeen described in detail in this disclosure, those skilled in the artwho review this disclosure will readily appreciate that manymodifications are possible in the exemplary embodiments (such asvariations in sizes, structures, shapes and proportions of the variouselements, values of parameters, or use of materials) without materiallydeparting from the novel teachings and advantages of the invention.Accordingly, all such modifications are intended to be included withinthe scope of the invention as defined in the appended claims. Othersubstitutions, modifications, changes and omissions may be made in thedesign, operating conditions and arrangement of the preferredembodiments without departing from the spirit of the invention asexpressed in the appended claims.

As used herein and in the following claims, articles such as “the”, “a”and “an” can connote the singular or plural.

Carbon Nanotubes to Impart EMI Shielding to Commercial Coatings andFilms

Carbon nanotubes were formulated and over-coated with 4 types ofconformal coatings for coating printed circuit assemblies as recognizedby, e.g. military specification MIL-1-46058. These conformal coatingswere then applied on industry standard FR4 PWB substrates and comparedto a non-filled control. The conductivity and EMI shielding propertiesof the novel coating system were characterized. This provides aconceptual demonstration of how the instant invention can convert mostmajor types of commercially available coatings into an EMI shieldingcoating. The technical results and details are provided in the followingsection.

Summary of Test Results

Carbon nanotubes were formulated with each of four conformal coatings,representing all the major types (under military specificationMIL-1-46058). These resins were then coated on standard FR4 PCBsubstrates with corresponding controls. Free standing films were alsoformed to demonstrate the utility of nanotubes to impart EMI shieldingto other films and coatings. Conductivity and EMI shielding propertiesof the coatings were evaluated. Results show that a thin nanotubecomposite coatings and films provide EMI shielding. Successfuldemonstration of this concept provides a means of converting most majortype of commercially available conformal coatings into an EMI shield.

A variety of resins and nanotubes for compounding were obtained. Thesecoatings were cast thinly onto common FR4 PWB and shipped for EMItesting to an accredited test facility. The results from our firstseries of tests showed low levels of shielding (50% transmission, or 3dB), however, subsequent rounds of testing provided much higher levelsof shielding (>97%, or 15 dB). Equally important is that these coatingsare very thin and due to low loading levels of nanotube, most otherproperties of the resin and coatings are unchanged. No difference in theprocessing of the filled and unfilled resins or coatings was observed.In other work, films were tested.

Types of Conformal Coatings

The source of conformal coating materials used was limited to onemanufacturer. HumiSeal Inc was selected as our initial source of resinssince they represent a large part of the market and can provide all thetypes of coatings. One of each type of conformal coating was evaluated:

HumiSeal 1A37HV represented the UV curable epoxy coatings.

HumiSeal 1B73 represented the thermally cured acrylic coatings.

HumiSeal 1A20 represented the thermally cured polyurethane coatings.

HumiSeal 1C49 represented the thermally cured silicone coatings.

The above resins were formulated with both single walled nanotubes andmulti-walled nanotubes.

The Printed Circuit Board Substrate

The circuit board (e.g. printed wire board, printed circuit board)material used for the testing of this coating is commercially available60-mil FR4 single clad printed circuit board, made by American BoardCompany (Vestal, N.Y.). The boards had a one mil copper cladding thatwas removed by the board manufacturer.

Removal of this cladding resulted in a 59 mil thick board. The boarddimensions were 1 8″×24″ prior to being cut into 18″×12″ rectangles inorder to facilitate the handling of the boards and testing.

Dispersion of Nanotubes

Both SWNT's and MWNT's were compounded into the conformal coatings. Thenanotubes were purchased from several sources and experimented with eachto find those most suitable for this demonstration. The SWNT's used forpreparing the samples were obtained from Carbon Nanotechnologies Inc.These were used as delivered. The MWNT's were obtained from SunNanotech, and Deal International. Sonication of the samples was doneusing a Cole-Parmer ultrasonic homogenizer. Dispersion of the nanotubeswas enhanced by sonication of the nanotubes in solvents like toluene orxylene for two cycles of 250 seconds. The dispersed nanotubes wereseparated from the solvent by centrifuging. The supernatant solvent wasdecanted and the remaining nanotubes were ready to be combined with theresin.

Formulation of Resin and Casting

HumiSeal 1A20 was formulated with the dispersed carbon nanotubes andalso formulations of 1A20 were made with multi-walled carbon nanotubes.The single walled nanotubes were loaded at 0.05%, 0.1%, and 0.4%concentrations. The multi-walled nanotubes were loaded at 0.2%, 4%, and5% concentrations. Uniform blending of the nanotubes into the coatingmaterial was achieved with an ultrasonic homogenizer. Sonication of thesamples was done for two 250-second cycles. The formulated samples werecoated on 18″×12″ FR4 unclad PCB. Application of the coatings materialwas achieved using a # 28 FAUSTEL metering rod that gave a coatingthickness of about 2.8 mils. The freshly coated samples were then placedin a vented preheated oven (100° C.) for 1 hour. The cured samples werethen cut to 8″×8″ squares. Two thin silver electrodes were painted onopposite sides of each of the 8″×8″ panels. The conductivity of each ofthe panels was measured and these results are provided, along with theEMI test results in Table 1.

The free standing film of CP1 (shown in Table 2) resin was compoundedwith nanotubes and cast from NMP solutions (12% by weight ) onto glassplates. They were then dried at 160° C. for eight hours in vacuum. TABLE1 Shielding Effectiveness Results Shielding Effectiveness Test, dB, atSample Type Frequency and Resistivity 200 kHz 500 MHz 1.0 GHzIdentification Ohms/Sq. Thickness SE_(pw) SE_(m) SE_(pw) SE_(m) SE_(pw)SE_(m) HumiSeal >10¹² 1.0 mil 0.46 0.51 0.56 0.60 0.66 0.63 1A20 Control0.2% SWNT 1.2 × 10⁸ 1.0 mil 0.36 0.42 0.46 0.48 0.51 0.50 A 0.05% SWNT>10¹² 1.0 mil 0.61 0.71 0.76 0.75 0.82 0.76 B 4.0% MWNT >10¹² 1.0 mil1.1 1.2 1.2 1.3 1.4 1.3 C 5.0% MWNT 5.0 × 10⁸ 1.0 mil 1.3 1.3 1.4 1.31.6 1.4 D 0.4% SWNT 4.0 × 10⁵ 1.0 mil 3.0 3.1 3.2 3.1 3.4 3.2 A2 0.4%SWNT 6.4 × 10⁴  15 mil 2.4 2.3 2.8 2.6 2.9 2.8 A3 3-Layer >10¹², 10⁴,10⁸ 3.0 mil 12.6 12.5 12.8 12.8 13.0 12.6 B3SE_(pW)—plane wave shielding effectiveness; SE_(m)—magnetic waveshielding effectiveness

TABLE 2 Shielding Effectiveness Results from second round of ExperimentsShielding Effectiveness Test, dB, at Frequency Sample Type andResistivity 200 kHz 5.0 GHz 10.0 GHz Identification Ohms/Sq. ThickSE_(pw) SE_(m) SE_(pw) SE_(m) SE_(pw) SE_(m) 1 Layer 1A20 & 0.2% >10¹² 19.34 7.34 9.7 8.02 9.9 8.15 SWNT 1 Layer 1A20 & 4% 1.2 × 10⁸ 1 10.4 7.8710.9 8.21 11.1 8.31 MWNT 2 Layer 1A20 & 4%  1.0 × 10¹² 1 11.5 9.13 12.19.51 12.4 9.63 MWNT 2 Layer 1A20 & 0.2%  1 × 10⁹ 1 10.3 7.43 10.8 8.111.0 8.21 SWNT 2 layers 1A20 & 1.2 × 10⁹ 1 12.6 9.23 13.0 9.7 13.2 9.81MWNT and SWNT 3 layers MWNT, 4.0 × 10⁵ 3 15.1 9.86 15.6 10.3 15.9 10.5SWNT, MWNT 4 layers MWNT, 6.4 × 10⁴ 4 15.3 9.91 15.7 10.4 15.9 10.6SWNT, MWNT, SWNT 0.3% SWNT in Si- 8.93 × 10⁵  0.8 10.1 7.47 10.3 8.1610.5 8.21 DETA/Si-TMXDI Ceromer coating 0.3% SWNT in CP-1 7.9 × 10⁶ 0.048.78 7.63 9.26 8.07 9.37 8.15 Polyimide filmSE_(pw)—plane wave shielding effectiveness; SE_(m)—magnetic waveshielding effectiveness*Please Note:For samples #'s 1-7 the concentration of SWNT used was 0.4% and theconcentration of MWNT used was 4.0%.

EMI Shielding Test

The testing was performed at the California Institute of Electronics andMaterial Science otherwise known as CIEMS (Hemet, Calif.).

Instruments and Devices:

The instruments and devices used by CIEMS and are described as follows:signal generator model 8592B (50 MHz to 22 MHz) HP, analyzer model 8592B(9 kHz to 22 GHz), dual preamplifier model 8847F HP, oscilloscope model10-4540 HK with amplifier model 8347A HP, antennas (HP11968C, HP11966E,HP11966F, and dipole antenna set HP11966H), magnetic field pickup coilHP11966K, active loop H-field HP11966A, goniometer model 63-844 MI,barometer model 602650 SB.

Testing Standards:

The above test equipment meets the applicable NIST, ASTM, ASME, OSHA,and state requirements. The test equipment was calibrated to standardtraceable to the NIST. The calibration was performed per ISO 9001 §4.11, ISO 9002 § 4.10, ISO 9003 § 4.6, ISO 9004 § 13, MIL-I 45208,IEEE-STD-498, NAVAIR-17-35-MTL-1, CSP-1/03-93, and the instrumentmanufacturers' specifications. The tests conformed to the following teststandards: ASTM D4935, J.EEE-STD-299-1991, FED-STD-1037,MIL-STD-188-125A, MIL-STD-4610, and MIL-STD-462.

Test Conditions:

The test condition were as follows: t=22° C., RH=43%, P=101.4 kPa. Eachmagnitude of the plane wave (S_(pw)) and magnetic (SE_(m)) shieldingeffectiveness in these measurements (See Results and Observations) wasan average of six runs of the test specimens at each of the three testfrequencies. The experimental error evaluated by the partial derivativesand least squares methods does not exceed 6%.

Test Range:

Due to the brevity in time and the high cost associated with determiningshielding effectiveness over the frequency range of 200 MHz to 40 GHz itwas decided that only discrete frequencies would be tested in a morenarrow range. Specific frequencies chosen were 200 MHz, 500 MHz, and 1GHz ranges. These ranges were chosen because of two reasons. Thesefrequencies represented the low, middle, and high portion of theintended test range and cover the range where most consumer electronicsoperate.

The following assumption can be made: if EMI shielding is observed inthe low and middle ranges the probability of observing EMI shielding inall the intermediary ranges is very likely. The same assumption can alsobe made regarding the middle to high frequency ranges.

Experimental Matrix: Table 1

A total of 5 coating formulations were sent out for initial analysis.These consisted of 4 samples of resin 1A20 formulated with 0.05%, 0.1%,and 0.4% single-walled nanotubes. The other two samples consisted ofresin 1A20 formulated with 0.2% and 5% multi-walled nanotubes. The finalthickness of most of the coatings was approximately 1 mil thick. Onlyone coating was cast to have a final thickness of 15 mils, the purposebeing to determine if shielding effectiveness was thickness dependant.

Furthermore, a composite multi-layer conformal coating was fabricated todetermine whether shielding effectiveness could be further enhanced.This experiment demonstrated that a synergistic effect betweensingle-walled nanotubes and multi-walled nanotubes occurs when they areprepared as a multi-layer composite. Each layer has a differentelectrical resistance and was cast to about 1 mil each. This resulted ina relatively thin three mils coating with good shielding.

Technical Aspects

The results obtained in the above work clearly demonstrate technicalfeasibility by accomplishing at least the following objectives:

Compounded SWNT's and MWNT's into a series of commercial conformalcoatings formulated from a wide range of polymer chemistries, such asurethane (thermal and UV curing), epoxy, acrylate, polyimide, andsilicone.

Demonstrated that thin free standing films exhibit the same shielding.

Prepared uniform thin coatings on FR4 PWB substrates.

Demonstrated EMI shielding of 15 dB with very low loading levels ofnanotubes and over 32 dB (or 99.9%) for 4 um thick monolayer of 50% SWNTand polymer binder.

Determined that thicker coating do not necessarily have higher shieldingefficacy for these nanocomposites.

Demonstrated that layers are an effective means of increasing shieldingeffectiveness.

Identified important direction for future development of thesenanocomposite coatings.

Observed no major technical road-blocks to the development of thistechnology.

These results provide strong evidence toward the use of carbon nanotubesin conformal coating to impart EMI shielding. These results also lead usto many questions as to the exact mechanism and how, if any synergyoccurs between layers of different nanotubes. We also observed non-ohmicbehavior in these coatings, with the electrical resistance dependant onthe test voltage. The lower the test voltage the higher the electricalresistance, and the higher the test voltage the lower the measuredresistance. This might not be surprising if one assumes that themechanism of electrical conduction between tubes is by electron hoppingor tunneling. It should also be noted that nanotube composites are alsobeing investigated for use in magnetic shielding, which leads to thepossibility of a second mechanism of EM absorption, since shielding canoccur by absorbing or reflecting the electric or magnetic fieldsundulating in EM radiation.

Accordingly, a polymer processing technique for incorporating nanotubesinto conformal coatings was demonstrated. This is a low cost approach toimparting EMI shielding characteristics to almost any conformal coatingtype. This approach yields a material which provides good EMI shieldingin a very thin coating.

If existing polymers were to be developed into conformal coatings theprocess could take years of research (and millions in cost). This newtype conformal coating would also require that application equipment bere-designed which could also run in the millions of dollars. Even afterall the above is done the probability is low that these coatings willmeet the barrier properties required for all military and commercialapplications. In the end if these EMI shielding polymer systems aredeveloped, the scope of use will most likely be very limited.

Presently there are no commercially available non-conducting conformalcoatings that provide EMI shielding. The instant inventors havedeveloped a conformal coating which when formulated with commerciallyavailable conformal coatings will provide EMI shielding properties inthe 30-50 dB attenuation range. Nanotubes are added in such smallamounts (≦0.05% by weight) that coating properties such as mechanicals,viscosity, and cost are not appreciably affected. Using commerciallyavailable conformal coatings formulated as described herein means thatlittle or no reformulation of the resin system is required. This alsomeans that little or no modification of the application techniques orequipment is required. In a short time and at a fraction of the cost alltypes of conformal coatings can be formulated as described herein tofulfill EMI shielding requirements. This innovation will broaden thescope of use of existing conformal coatings that in turn will changemilitary and civilian structures as they are known today.

Preparation of Spray Coatings

The second series of coating on FR4 were prepared using the proceduredescribe below, to yield a highly concentrated layer of nanotube withvery low resistivity.

Advanced Nanotube Coating Procedure:

Apply HumiSeal 1A20 conformal coating cast onto to fiberglass FR4circuit board;

Coating dried in an oven to remove solvents;

Spray coat a mixture of nanotubes in solvents plus a small amount ofconformal coating;

Coating is dried in oven; and

If more protection of the coating is preferred, a thin layer of virginconformal coating is applied, with a drying step to follow.

The first layer applied was a virgin conformal material coating sprayedonto the FR4 circuit board using conventional air spraying techniquesand film casting as described above. This layer forms an insulatinglayer between the board (also components and conductors if populated)and the nanotube coating. After a drying step, the next process step wasto spray coating of nanotubes over this layer. The single-walled carbonnanotubes were first exfoliated in toluene by sonicating techniques.After the removal of most of the toluene, a mixture of solvents wasadded (propylene glycol methyl acetate, xylene and ethyl benzene) to becompatible with the conformal coating.

This mixture of SWNT's and compatible solvents are applied through spraycoating techniques onto the circuit board. The adding of a small amountof conformal coating to this mixture can elevate the need of a finalprotective over coating and results in a conductive top coat forgrounding. Otherwise, a third very thin coating of virgin conformalcoating could be applied. By using the same solvents as the originalconformal coating, the nanotube coating will bond to the previous layerand interpenetrate. The solvents can be driven off relatively fast usingstandard oven drying in air. A series of these coating were prepared tospan a range of electrical resistivity and were tested for SE using thewave guide apparatus describe in the next section.

EMI Shielding Test Apparatus and Procedures

EMI shielding effectiveness testing was conducted using both free spaceand wave guide methods as describe below.

The test equipment used in these EXAMPLES meets the applicable NIST,ASTM, ASME, OSHA, and state requirements. The test equipment wascalibrated to standard traceable to the NIST. The calibration wasperformed per ISO 9001 § 4.11, ISO 9002 § 4.10, ISO 9003 § 4.6, ISO 9004§ 13, MIL-I 45208, IEEE-STD-498, NAVAIR-17-35-MTL-1, CSP-1/03-93, andthe instrument manufacturers' specifications. The tests conformed to thefollowing test standards: ASTM D4935, J.EEE-STD-299-1991, FED-STD-1037,MIL-STD-188-125A, MIL-STD-4610, and MIL-STD-462.

Wave guide Testing:

Engineering Specialties Service (ESS) in East Bridgewater, MA wascontacted to perform wave guide measurements. Due to the flat SEresponse of carbon nanotubes the frequency dependence is slight,allowing for fixed frequency analysis. The Wave guide test operates at afix frequency by design and allows for very accurate testing of SE andother dielectric properties of materials. The method used a wave guidecavity where the material was measured at x-band frequencies. Thefrequencies tested were from 7.0 GHz to 12 GHz. All of the recordedreadings were made at 10 GHz. The test consists of an RF input microwavesweeper, reference sensor, VSWR test channel, and insertion loss/phasetest channel. Measurements made using the same coating on both freespace and wave guide apparatus provide similar results.

Experimental Test Matrix and Results from SE Testing

The experimental test matrix is divided into two groups: 1) coatingsprepared by compounding or dispersing the nanotubes into the conformalcoating (discussed above) and 2) coatings prepared by forming aninterpenetrated network of nanotubes on an unfilled conformal coatingand then by sealing/binding with the conformal coating. Each of thesegroups was evaluated for SE, electrical resistivity, cure, andqualitative processing characteristics in comparison to unmodifiedconformal resin. SE testing was conducted by free space testinginitially on all samples, however to speed development and evaluation weswitched to a wave guide test which allowed much faster evaluation andfeedback. Consequently, the final series of testing, which yielded themost impressive SE results, were entirely conducted using the moreaccurate fixed frequency wave guide test. The preparation of thesecoatings is describe in previous sections.

Results: Conformal Coatings with IPN of Nanotubes

The data presented here better demonstrates how carbon nanotubes can beutilized to impart high shielding effectiveness to literally any resin,including conformal coatings. The application technique employed toprepare these coatings allows for the concentration of nanotubes into ahighly conductive layer within any protective conformal coating to yieldan effective EMI field using traditional processing technology. The testresults were selected from a larger set of samples to fully representthe range of results obtained for a wide range of coating resistivitylevels. As can be seen, using this coating technique coating can beproduced with a very wide range of electrical resistivity. In fact,coated specimens were made with much higher resistivity (turnable from10⁰ to 10¹² Ohms/Square) although many are not shown since those withhigher resistivity exhibit lower SE. Coatings with resistivity higherthan ˜100 Ohms are also transparent to visible light, and are nearlycolorless to the eye in coatings with resistivity higher than ˜1000Ohms. These observations have lead to the evaluation of these coatingsfor numerous other applications. TABLE 3 Shielding Effectiveness TestResults from Spray Coated Nanotubes Forming a IPN with HumiSeal 1A20Urethane Conformal Coating Sample Ohms/ Thickness Measured Measured #Description Square Mils* VSWR SE (db) 1 Virgin FR4 Board >10¹² 0 2.120.5 2 SWnT coated 2.6 × 10³ <0.1 4.19 8.8 3 SWnT coated 820 <0.1 4.218.2 4 Purified SWnT 221 <0.1 5.84 10.0 5 Purified SWnT 46 0.16 >20 20.6Thick 6 D/V SWnT 2.0 0.59 0.38 28.9 7 E/V SWnT 2.0 0.43 0.41 38.6 8 E/VSWnT 1.9 0.43 0.21 35.5 9 E Silver 0.02 3 0.41 54.0*Thickness in 1/1000 inches for the active layer containing nanotubes**The detector level of sensitivity and calibration limit the measuredSE to a <55 dB max value.

The above demonstrates EMI shielding of ˜40 dB with very thin coatingsof nanotubes layered in the conformal coating resin to form ainterpenetrated network of tubes within the resin. The above alsodemonstrates that layers are an effective means of increasing shieldingeffectiveness.

All references cited herein, including all U.S. and foreign patents andpatent applications, as well as any other documents or referencematerials cited herein, are specifically and entirely herebyincorporated by reference. It is intended that the specification andexamples be considered exemplary only.

1-32. (canceled)
 33. A method for imparting EMI shielding to asubstrate, comprising coating said substrate with a conformal coatingwherein said conformal coating comprises: an insulating layer, and ananotube-containing layer disposed on said insulating layer, whereinsaid nanotube-containing layer comprises a plurality of carbonnanotubes.
 34. The method of claim 33, wherein the substrate is part ofa device component selected from the group consisting of keypads,catheters, integrated circuits, printed circuit boards, printed wireboards, hybrids, transducers, sensors, cores, accelerometers, catheters,coils, fiber optic components, heat exchangers, pacemakers, implants,flow meters, magnets, photoelectric cells, electrosurgical instruments,and plastic encapsulated microcircuits.
 35. The method of claim 33,wherein the insulating layer comprises a material selected from thegroup consisting of polyurethanes, parylene, acrylics, expoxy andsilicone.
 36. The method of claim 33, wherein the carbon nanotubes areselected from the group consisting of single-walled nanotubes,double-walled nanotubes, multi-walled nanotubes, and mixtures thereof.37. The method of claim 33, wherein the carbon nanotubes aresubstantially single-walled nanotubes.
 38. The method of claim 33,wherein the nanotube-containing layer further comprises a polymericmaterial, wherein the polymeric material comprises a material selectedfrom the group consisting of thermoplastics, thermosetting polymers,elastomers, conducting polymers and combinations thereof.
 39. The methodof claim 33, wherein the carbon nanotube-containing layer furthercomprises a polymeric material, wherein the polymeric material comprisesa material selected from the group consisting of polyethylene,polypropylene, polyvinyl chloride, styrenic, polyurethane, polyimide,polycarbonate, polyethylene terephthalate, cellulose, gelatin, chitin,polypeptides, polysaccharides, polynucleotides and mixtures thereof. 40.The method of claim 33, wherein the carbon nanotube-containing layerfurther comprises a polymeric material, wherein the polymeric materialcomprises a material selected from the group consisting of ceramichybrid polymers, phosphine oxides and chalcogenides.
 41. The method ofclaim 33, wherein the carbon nanotube-containing layer further comprisesa conformal coating material selected from the group consisting ofpolyurethanes, parylene, acrylics, epoxies and silicone.
 42. The methodof claim 33, wherein the carbon nanotube-containing layer furthercomprises an additive selected from the group consisting of a dispersingagent, a binder, a cross-linking agent, a stabilizer agent, a coloringagent, a UV absorbent agent, and a charge adjusting agent.
 43. Themethod of claim 33, wherein the carbon nanotube-containing layer has atotal transmittance of at least about 60%.
 44. The method of claim 33,wherein the carbon nanotubes are oriented.
 45. The method of claim 33,wherein the coating further comprises an over-coat comprising aconformal coating material selected from the group consisting ofpolyurethanes, parylene, acrylics, epoxies and silicone.
 46. The methodof claim 33, wherein the carbon nanotube-containing layer provides EMIshielding properties in the 10-70 dB attenuation range.
 47. The methodof claim 33, wherein the carbon nanotubes are chemically modified.
 48. Aconformal coating that provides EMI shielding, wherein said coatingcomprises a plurality of carbon nanotubes and a polymer selected fromthe group consisting of acrylics, epoxies, silicone, polyurethane, andparylene.
 49. The conformal coating of claim 48, wherein the carbonnanotubes are selected from the group consisting of single-wallednanotubes, double-walled nanotubes, multi-walled nanotubes, and mixturesthereof.
 50. The conformal coating of claim 48, wherein the carbonnanotubes are substantially single-walled nanotubes.
 51. The conformalcoating of claim 48, wherein the coating has a surface resistance in therange of less than about 10⁴ ohms/square.
 52. The conformal coating ofclaim 48, wherein the film has a surface resistance in the range of lessthan about 10³ ohms/square.
 53. The conformal coating of claim 48,wherein the film has a surface resistance in the range of about 10⁻²-10⁰ohms/square.
 54. The conformal coating of claim 48, wherein the coatingprovides EMI shielding properties in the 10-70 dB attenuation range. 55.The conformal coating of claim 48, wherein the carbon nanotubes arechemically modified.
 56. The conformal coating of claim 48, wherein thecarbon nanotubes are present on a surface of said conformal coating. 57.A dispersion comprising a plurality of carbon nanotubes and a conformalcoating material selected from the group consisting of polyurethanes,parylene, acrylics, epoxies and silicone.
 58. The dispersion of claim57, wherein the carbon nanotubes are selected from the group consistingof single-walled nanotubes, double-walled nanotubes, multi-wallednanotubes, and mixtures thereof.
 59. The dispersion of claim 57, whereinthe carbon nanotubes are substantially single-walled nanotubes.
 60. Thedispersion of claim 57, further comprising a polymeric material, whereinthe polymeric material comprises a material selected from the groupconsisting of thermoplastics, thermosetting polymers, elastomers,conducting polymers and combinations thereof.
 61. The dispersion ofclaim 57, further comprising a plasticizer, softening agent, filler,reinforcing agent, processing aid, stabilizer, antioxidant, dispersingagent, binder, a cross-linking agent, a coloring agent, a UV absorbentagent, or a charge adjusting agent.
 62. The dispersion of claim 57,further comprising conductive organic particles, inorganic particles orcombinations or mixtures thereof.
 63. The dispersion of claim 62,wherein the conductive organic particles are selected from the groupconsisting of buckeyballs, carbon black, fullerenes, and combinationsand mixtures thereof.
 64. The dispersion of claim 62, wherein theconductive inorganic particles are selected from the group consisting ofnickel, silver and copper.
 65. The dispersion of claim 62, wherein thedispersion can form a coating, wherein the coating provides EMIshielding properties in the 10-70 dB attenuation range.
 66. Theconformal coating of claim 48, wherein the carbon nanotubes aresingle-wall carbon nanotubes and the EMI shielding is from 10-70 dBattenuation range.